This project is designed to develop a means of replacing radioiodine, the radionuclide presently employed in the labeling of antibodies, with a radionuclide that reduces the absorbed radiation dose to the patient and has physical properties that can be more efficiently utilized by modern radiation detecting systems, specifically the gamma camera. During the past year, we have developed a procedure for labeling antibodies with indium III. This procedure involves the conjugation of the IgG fraction of the antisera with iron-free transferrin using glutaraldehyde as the coupling compound. Indium III ions are then added to the solution containing the transferrin-IgG conjugate and are instantaneously chelated by transferrin. The projected goals for this year include the refinement of the conjugating procedures and the evaluation of the effectiveness of intravenously injected III In-transferrin-anti-CEA conjugate obtained from antisera raised in goats in detecting colonic tumors transplanted in animals. The results will be compared with those obtained with anti- CEA preparations labeled with I131 in the standard manner.